In this study the possibility for the improvement of accuracy of single wavelength measurements organized by multiple angle of incidence (MAI) null ellipsometry technique is shown. The “transparent film on transparent substrate” system in a range of 1.0–20.0 nm film thickness in the low-contrast region on the film-substrate surface was studied. The developed method allows an independent determination ofthe thickness and refractive index of an ultrathin transparent film under conditions of a strong correlation effect between these parameters.